<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			.box1 {
				border: 5px solid red;

				/* 内容的宽度 */
				width: 300px;

				height: 300px;

				/* 内容和边框的距离 */
				padding: 20px;
			}

			.box2 {
				border: 5px solid blue;
				/* width: 500px; */
				height: 290px;
			}

			.box3 {
				border: 5px solid green;
				height: 280px;
			}
		</style>
	</head>
	<body>
		<div class="box1">
			<div class="box2">
				<div class="box3"></div>
			</div>
		</div>
	</body>
</html>
